Porous coordination polymers (“PCP”) are a class of hybrid inorganic-organic, typically crystalline materials (amorphous PCPs are also known) whose structure and properties can be rationally tailored by the selection of their component chemical moieties. Distinguishing features of a PCP are coordinating metallic groups causing organic ligands to self-organize into two- or three-dimensional open-pore structures. These structures retain their porosity upon removal of “guest” molecules (e.g., a solvent or other similar molecule), enabling them to serve as reversible sorbents for a variety of molecular species. Two broad subcategories of PCPs reported in the literature include, but are not limited to, metal-organic frameworks (MOFs) and zeolitic imidazolate frameworks (ZIFs). Covalent organic frameworks (COFs) are a related class of porous materials, in which the nanoporous structure comprises a network of main-group atoms (typically silicon and/or boron) covalently bonded to organic linking groups. Examples of PCPs include the series of structures known as iso-reticular MOFs (IRMOFs), composed of zinc ions coordinated to carboxylate anions, and the so-called MIL-series of iron-, chromium-, and aluminum carboxylates (MIL stands for “Materials of Institut Lavoisier,” a research center associated with Versailles Saint-Quentin-en-Yvelines University, France). Other well-known examples of PCPs include, but are not limited to, CuBTC (also known as HKUST-1), which is short-hand nomenclature for copper (II) benzene-1,3,5-tricarboxylate [chemical formula Cu3(BTC)2(H2O)3], NOTT-100 (biphenyl-3,3′,5,5′-tetracarboxylic acid), NOTT-101 (terphenyl-3,3′,5,5′-tetracarboxylic acid), and ZIF-8 (Zn(mim)2.2H2O (where mim stands for methylimidazole).
PCP coatings on sensors possess a number of features that make them potentially superior to polymers and other coatings currently used to impart sensitivity and selectivity to chemical sensors. Firstly, PCP crystal structures and the chemical makeup of their pore structure can be tailored so that they have high selective affinity for a variety of analyte species. This contrasts with other nanoporous materials, such as zeolites, aerogels, synthetic opals, and nanotubes (both carbon- and non-carbon based). Secondly, PCPs have been observed to possess BET surface areas of up to 7000 m2/g, a result approximately seven times greater than a zeolite. This makes these materials potentially highly effective sorbents that would increase sensitivity in sensors detecting either mass or stress changes. Third, the ability to tailor PCP pore sizes by changing the chemical nature of the linking molecule, changing the metal center, or changing the pore geometry, enables both the chemical selectivity and adsorption properties to be optimized in a rational way. Fourth, effective mechanical linkages between PCP crystalline or amorphous layers and a substrate can be created by covalent bonding schemes that anchor the PCP to the sensor surface. These qualities result in robust, stable sensors capable of generating large signals from small quantities of targeted analytes.